The adaptations in extracellular signal-regulated kinase (ERK) pathway activity result in alterations in transcription of several genes that can be essential for development of both opioid tolerance and dependence. In this study, we investigated the effect of acute and prolonged opioid treatment on ERK pathway activity in SH-SY5Y cells. Acute administration of morphine and DAMGO stimulated ERK activity and this stimulation required activation of Ca(2+)/calmodulindependent kinase II (CaMKII) and protein kinase C (PKC). In contrast, prolonged morphine treatment decreased the level of phosphorylated ERK. The pr ecipitation of withdrawal further decreased the ERK1/2 activity. The principal finding of these studies is demonstration that the activation of CaMKII and PKC is required for ERK stimulation following acute opioid treatment while in a chronic morphine treatment and withdrawal, the up-regulation of PKC and CaMKII pathways seems to be engaged in the ERK inhibition. These results provide evidence that both opioid administration and opioid withdrawal, affecting similar intracellular pathways, can exert different effects on ERK activity.